1. Field of the Invention
This invention relates to a flexible meshing type gear device, particularly to the shape of the teeth of a rigid internal gear and a flexible external gear used in the device.
2. Background Art
From the time of the invention of the flexible meshing type gear device by C. W. Musser (U.S. Pat. No. 2,906,143) up to the present, Musser and many other researchers, including the inventor of the invention described in this specification, have proposed various innovations. The variety of improvements in tooth profile proposed alone is considerable.
Regarding the tooth profile of the flexible meshing type gear device, the present inventor earlier proposed a tooth profile design method that uses rack-approximation of meshing between the teeth of the rigid internal gear and the flexible external gear to derive a deddendum profile enabling wide range tooth meshing between the two gears (see JP-B 45-41171). The inventor also invented a method for avoiding tooth profile interference caused by rack approximation (JP-A 7-167228).
There is a strong demand in the market for flexible meshing type gear devices that offer better performance at a lower price. On the performance side, a particular need is felt for improvement in load capacity. The main factors governing the load capacity of a flexible meshing type gear device are the rim of the flexible external gear tooth root and the rolling contact surface of the inner ring of the wave generator, particularly in the vicinity of the major axis.
One object of this invention is to provide a flexible meshing type gear device whose performance is enhanced by reducing the stress produced by these two factors. Another object of this invention is to provide a flexible meshing type gear device that is also low in machining cost, including tool cost, owing to the adoption of an involute curve as a basic tooth profile.
The present invention achieves the foregoing object by adopting negative deflection that makes the flexing of the flexible gear smaller than standard and separates most of tooth meshing region from the vicinity of the major axis of the ellipse, thereby preventing superimposition of the bending stress produced in the vicinity of the major axis by the elliptical deformation of the rim of the flexible external gear and the tensile stress produced in the rim by tooth flank load, and further by uniformly distributing the spherical load of the wave generator.
In order to reduce machining cost, this invention, concurrently with the foregoing, also adopts an involute tooth profile as the basic tooth profile to apply the Euler-Savari equation that holds for tooth meshing in speed change gears and thus determine the tooth profile specifications to ensure meshing continuity.
Specifically, the present invention provides a flexible meshing type gear device having a rigid internal gear, a flexible external gear disposed inside the internal gear and a wave generator for flexing the external gear into an elliptical shape to cause the flexible external gear to mesh with the rigid internal gear at extremities of the major axis of the elliptical shape and rotate the two meshing positions in the circumferential direction,
which flexible meshing type gear device is characterized in that:
the rigid internal gear and the flexible external gear are both basically spur gears,
the number of teeth of the flexible external gear is 2n (n being a positive integer) fewer than the number of teeth of the rigid internal gear,
a cross-section taken perpendicular to the axis of the flexible external gear at an arbitrarily selected location in the direction of the tooth trace is defined as a main cross-section,
a position spaced from the major axis of the neutral curves of the elliptical rim of the flexible external gear in the main cross-section by an arbitrarily selected displacement angle of not greater than 45 degrees is defined as the tooth meshing center position,
the basic shapes of the tooth profiles forming the main portion of the mesh of the two gears are defined as involute curves,
one circle of curvature at the point of contact of the involute curve making contact at the meshing center position is made coincident with a tooth profile circle of curvature satisfying the Euler-Savary equation that applies between the tooth profile curvatures at the tooth profile contact points of speed change gears, and
another tooth profile circle of curvature of the equation is defined in the vicinity of the circle of curvature of another involute curve.
The tooth crests of the two gears are preferably modified to form a continuously meshed state of the teeth at positions spaced from the major axis in a section perpendicular to the axis of the flexible external gear.
Known flexible external gears include cup-like and silk-hat-like flexible external gears that comprise a cylindrical body, a diaphragm extending radially inward from one end of the cylindrical body, and external teeth formed on an opening portion at the other end of the cylindrical body. When this type of flexible external gear is used, the wave generator elliptically flexes cross-sections of the flexible external gear perpendicular to its axis so that the amount of flexing increases from the diaphragm toward the opening portion approximately in proportion to the distance from the diaphragm.
In this case, it is preferable to modify the tooth crests of both gears and/or to apply relieving to the teeth from the main cross-section of the flexible external gear toward the opening portion and toward the diaphragm side in order to form a continuously meshed state of the teeth at positions spaced from the major axis in sections perpendicular to the axis of the flexible external gear in a region centered on the main cross-section.
In an embodiment of the present invention, the main portion of the tooth profile of the rigid internal gear is defined as an involute curve and the tooth profile of the flexible external gear at the main meshing position in the main cross-section is defined as a circle of curvature determined by the Euler-Savary equation that holds in speed change gears.